Method and apparatus for decoding bar code symbols using subpixel scan lines

ABSTRACT

A bar code reader decodes a bar code symbol in a pixel image by transforming the data corresponding to the symbol along a first scan line, where the transformation comprises rotation and stretching. The reader generates a composite vector and a count vector from the transformed data. The reader then transforms (by rotating and stretching) another set of data corresponding to the symbol along a second scan line. The reader updates the composite and count vectors using the second set of transformed data. The reader then generates a one-dimensional composite signal from the composite and count vectors. The reader decodes the symbol by decoding the composite signal.

This is a divisional of copending application Ser. No. 08/011,642 filedon Jul. 29, 1993 now U.S. Pat. No. 5,384,451.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to image processing, and, in particular,to a method and apparatus for locating and decoding bar code symbols inpixel images.

2. Statement of Related Art

Reading bar code symbols with one-dimensional laser scanning systems iswell known. Less well known are image processing systems for locatingand decoding bar code symbols in two-dimensional pixel images. Decodingbar code symbols requires accurate measurement of edge-to-edgedistances, where an edge corresponds to a transition from a bar to aspace or from a space to a bar within a bar code symbol.

Of particular difficulty is the reading of high-density bar code symbolsin facsimile-quality images, that is, images of bar code symbolsgenerated by facsimile machines and the like. A bar code reader that canlocate and decode bar code symbols in an image generated by a facsimilemachine should be able to be adapted to read bar code symbols in animage generated by any other image source such as a scanner or a camera.

SUMMARY OF THE INVENTION

In a preferred embodiment, the present invention is a method andapparatus for reading a bar code symbol in a pixel image. According tothis embodiment, a first scan line substantially parallel to the barsand spaces of the symbol is selected. The symbol is then scanned alongthe first scan line to generate a first value. A second scan linesubstantially parallel to the bars and spaces of the symbol is thenselected, where the distance between the first and second scan lines isless than the width of a pixel of the image. The symbol is then scannedalong the second scan line to generate a second value. The symbol isthen decoded in accordance with the first and second values.

In an alternative preferred embodiment, the present invention is amethod and apparatus for reading a bar code symbol in a pixel image.According to this embodiment, a first scan line crossing at least afirst portion of the symbol is selected. The first portion is thentransformed by rotating the first portion to an angle substantiallyparallel to either a row or column of pixels in the image and stretchingthe first portion. A composite signal is generated in accordance withthe transformed first portion. A second scan line crossing at least asecond portion of the symbol is selected. The second portion is thentransformed by rotating the second portion to an angle substantiallyparallel to either a row or column of pixels in the image and stretchingthe second portion. The composite signal is updated in accordance withthe transformed second portion and the symbol is decoded in accordancewith the composite signal.

In another alternative preferred embodiment, the present invention is amethod and apparatus for reading a bar code symbol in a pixel image.According to this embodiment, the symbol is transformed by rotating thesymbol to an angle substantially parallel to either a row or column ofpixels in the image and stretching the symbol. First and second scanlines substantially perpendicular to the bars and spaces of thetransformed symbol are selected. A composite signal is then generated inaccordance with the first and second scan lines and the symbol isdecoded in accordance with the composite signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of a bar code symbol reading systemaccording to a preferred embodiment of the present invention;

FIG. 2 is a block flow diagram of processing implemented by the symbollocator of the system of FIG. 1 to locate bar code symbols in a pixelimage;

FIG. 3 is a block flow diagram of processing implemented by the symbollocator of FIG. 2 to locate candidate bar code symbols in binary images;

FIG. 4 is an image of a bar code symbol located by the symbol locator ofFIG. 2;

FIG. 5 is an image of a bar code symbol in which the trailing bar isbroken into two segments;

FIG. 6 is a block flow diagram of a slow method implemented by apreferred embodiment of the composite signal generator of the system ofFIG. 1 for generating a composite signal of a bar code symbol;

FIG. 7 is an image of a bar code symbol showing the sample rectangleover which the symbol is analyzed by the composite and gradient signalgenerators of the system of FIG. 1;

FIG. 8 is an image of a portion of the bar code symbol of FIG. 7 showingsome of the scan lines used by the composite signal generator of thesystem of FIG. 1 in implementing the slow method of FIG. 6;

FIG. 9 is a block flow diagram of a slow method implemented by apreferred embodiment of the composite signal generator of the system ofFIG. 1 for generating a composite signal of a bar code symbol;

FIG. 10 is an image of the bar code symbol of FIG. 7 showing scan linesused by the composite signal generator of the system of FIG. 1 inimplementing the fast method of FIG. 9;

FIG. 11 is a block flow diagram of the processing implemented by thecomposite signal thresholder of the system of FIG. 1 to threshold thecomposite signal of a bar code symbol;

FIG. 12 depicts the binary representation of part of a bar code symbolsignal with a spurious bar;

FIG. 13 depicts the binary representation of part of a bar code symbolsignal with either a spurious bar or a spurious space;

FIG. 14 is a block flow diagram of the processing implemented by thecomposite signal decoder of the system of FIG. 1 to decode thecharacters of a Code 39 bar code symbol;

FIG. 15 is a block flow diagram of the processing implemented by thegradient signal generator of the system of FIG. 1 to generate whiteningand blackening gradient signals for a bar code symbol; and

FIG. 16 is a block flow diagram of the processing implemented by thegradient signal processor of the system of FIG. 1 to process whiteningand blackening gradient signals.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is shown a functional block diagram of barcode symbol reading system 100 according to a preferred embodiment ofthe present invention. System 100 locates and decodes bar code symbolswith unknown positions and orientations in two-dimensional pixel images.System 100 may be designed to locate and decode bar code symbols ineither binary images or gray-scale images.

Data input 102 of system 100 receives the data for a pixel image andsymbol locator 104 locates bar code symbols contained in the pixelimage. System 100 preferably attempts to decode each located bar codesymbol in the pixel image by two parallel processes.

According to one process, composite signal generator 106 generates acomposite signal corresponding to the bar code symbol. Composite signalthresholder 108 then thresholds and filters the composite signal togenerate a binary signal corresponding to the composite signal.Composite signal decoder 110 then decodes the bar code symbol bydecoding the binary signal. If signal decoder 110 successfully decodesthe bar code symbol, decoder 110 transmits the decoded signal to outputselector 118. Otherwise, signal decoder 110 transmits a signal to outputselector 118 indicating that the decoding was unsuccessful.

According to the other process for decoding each located bar codesymbol, gradient signal generator 112 generates two gradient signalsfrom the located bar code symbol. Gradient signal processor 114processes the two gradient signals to generate a binary reconstructedsignal. Gradient signal decoder 116 then decodes the bar code symbol bydecoding the reconstructed signal. If signal decoder 116 successfullydecodes the bar code symbol, decoder 116 transmits the decoded signal tooutput selector 118. Otherwise, signal decoder 116 transmits a signal tooutput selector 118 indicating that the decoding was unsuccessful.

In a preferred embodiment, composite signal decoder 110 and gradientsignal decoder 116 perform identical decoding algorithms on the binarysignals received from composite signal thresholder 108 and gradientsignal processor 114, respectively.

Output selector 118 receives the decoded signals from signal decoders110 and 116 and selects the appropriate output for transmission to dataoutput 120. If either or both of signal decoders 110 and 116successfully decode the bar code symbol, then output selector 118transmits the decoded signal to data output 120. Otherwise, the bar codesymbol was not decoded and output selector 118 transmits an appropriatesignal to data output 120.

Locating Bar Code Symbols

Referring now to FIG. 2, there is shown a block flow diagram for theprocessing implemented by symbol locator 104 of system 100. Symbollocator 104 locates a bar code symbol in a pixel image by searching fora bar code symbol quiet zone that is followed by a minimum number oftransitions between symbol bars and spaces. On each end of a bar codesymbol in a pixel image is a region of bright pixels called a quietzone. The symbol itself is comprised of a sequence of dark barsseparated by bright spaces.

According to a preferred embodiment, means 202 of symbol locator 104selects a new search line in the pixel image. A search line may beeither a row or column of the pixel image. Means 202 may select searchlines employing a binary search such as that described U.S. patentapplication Ser. No. 07/927,910, U.S. Pat. No. 5,343,028 entitled"Method and Apparatus for Detecting and Decoding Bar Code Symbols,"filed on Aug. 10, 1992, which is commonly owned by the present assigneeand the disclosure of which is incorporated herein in its entirety byreference. A suitable system for selecting search lines using a binarysearch is described in the section of the '910 application entitled"Detecting and Decoding Bar Code Symbols." In general, since a bar codesymbol typically spans multiple rows and columns in the pixel image,means 202 preferably selects rows in a sequence employing a broadpattern that gets finer as processing proceeds.

Table I presents a preferred sequence for selecting rows (or analogouslycolumns) in a pixel image having, for example, 2048 columns and 4096rows. In the preferred sequence, the selection of rows as search linesstarts at row number 64 with a step size of 64. After selecting rows(64, 128, 192, . . . , 4032, 4096), the sequence returns to row number32 with the same 64-row step size. The selection continues as indicatedin Table I. After selecting rows (4, 12, 20, . . . , 4084, 4092), theentire sequence will have selected every fourth row in the image withoutrepeating any row twice.

                  TABLE I                                                         ______________________________________                                        Row Step    Starting Row                                                                             Sequence of                                            Size        Number     Rows                                                   ______________________________________                                        64          64         (64, 128,. . . ,4096)                                  64          32         (32, 96,. . . ,4064)                                   32          16         (16, 48,. . . ,4080)                                   16          8           (8, 24,. . . ,4088)                                   8           4           (4, 12,. . . ,4092)                                   ______________________________________                                    

A sequence of selecting search rows such as that in Table I typicallyincreases the speed and efficiency of system 100. Those skilled in theart will understand that such a sequence may be varied depending on thecharacteristics of the pixel images to be processed.

In a preferred embodiment, if the pixel image contains a known number ofbar code symbols, the search sequence is terminated after the requisitenumber of bar code symbols have been located and decoded. In addition,system 100 preferably keeps track of the areas of the image that containbar code symbols that have already been located and decoded. Symbollocator 104 preferably ignores those areas that have already beenprocessed.

Means 204 begins searching along a selected search line for a candidate(or potential) bar code symbol from a starting edge of the pixel image.Where the bar code symbols expected to be found in the pixel image havea known minimum length, means 204 preferably does not search the entirelength of each selected search line. Since system 100 decodes only"whole" bar code symbols, means 204 preferably ignores a portion of eachselected search line adjacent to the stopping edge of the search line.The portion of each search line that is ignored corresponds in distanceto the shortest expected bar code symbol.

Means 204 recognizes a candidate bar code symbol as a continuoussequence of N1 "bright" pixels (corresponding to an expected quiet zone)followed by a sequence of N2 transitions between "bright" and "dark"pixels (corresponding to transitions between expected bars and spaces),where N1 and N2 are first and second specified thresholds, respectively.

Means 204 may search for candidate bar code symbols in either binary orgray-scale images. In binary images, bright pixels may be defined tohave a value of 1 and dark pixels may be defined to have a value of 0.In gray-scale images, a pixel having a value greater than a thirdthreshold may be defined to be a bright pixel; otherwise, it is a darkpixel.

Means 204 functions by first searching for a candidate bar code symbolquiet zone. Means 204 detects a candidate bar code symbol quiet zonewhen it finds a continuous sequence of N1 bright pixels along theselected search line. Once a candidate bar code symbol quiet zone hasbeen detected, means 204 next searches along the selected search linefor a sequence of N2 bright/dark transitions, where each bright/darktransition corresponds to an edge of a bar in the candidate bar codesymbol (i.e., a transition between a bar and a space of the symbol). Theterm "bright/dark transition" refers collectively to both bright-to-darktransitions and dark-to-bright transitions.

After locating the first dark pixel following a candidate quiet zone,means 204 checks whether that dark pixel is contained in a bar codesymbol that has already been located and decoded by system 100. If so,then the candidate bar code symbol is rejected, since it has alreadybeen processed.

In addition, while counting the number of bright/dark transitions, means204 checks the width of each candidate bar and space. If any candidatebar/space is too wide (i.e., exceeds a specified maximum bar/spacethreshold), then the candidate symbol is rejected and a new search for acandidate bar code symbol quiet zone is started.

After locating a candidate bar code symbol along a selected search line(i.e., after locating a candidate quiet zone followed by a minimumnumber of candidate bars and spaces), means 204 verifies the candidatesymbol by repeating the quiet-zone searching and bar/space transitioncounting along one or more neighboring rows/columns in the pixel image.

Byte-Based Searching of Binary Images

Referring now to FIG. 3, there is shown a block flow diagram of theprocessing implemented by means 204 of symbol locator 104 to locatecandidate bar code symbols when the pixel image is a binary image. In abinary image, each pixel is represented by a single bit, where, forexample, a "1" corresponds to a bright pixel and a "0" corresponds to adark pixel. In a preferred embodiment, means 204 performs byte-basedsearching for quiet zones and byte-based counting of bar/spacetransitions, where eight consecutive binary pixels in the pixel imageare treated as a single byte of image data.

Means 302 searches along the selected search line one byte at a time. Abyte corresponding to eight pixels all of which are located in acandidate quiet zone will typically have all eight bits (i.e., pixels)equal to 1. Thus, if the minimum quiet zone length is equivalent to 32pixels, then a search line through a quiet zone will have at least threeconsecutive bytes that are all 1's. If means 302 detects threeconsecutive quiet-zone bytes (i.e., all 1's), then a candidate quietzone is located and means 304 directs processing to continue to means306. Otherwise, processing returns to means 302 to continue byte-basedsearching for a candidate quiet zone.

Means 306 performs byte-based counting of bar/space transitions in thecandidate bar code symbol. Each byte that represents bars and spaces ina candidate bar code symbol provides specific information regarding thenumber of bright/dark transitions in a portion of the symbol. Forexample, the byte "01100110" contains four transitions--two from dark tobright (i.e., from 0 to 1) and two from bright to dark (i.e., from 1 to0). Moreover, if the last bit from the previous byte was "1," then abright-to-dark transition exists between the previous byte and thecurrent byte. Similarly, if the first bit in the next byte is a "1,"then a dark-to-bright transition exists between the current byte and thenext byte.

Means 306 preferably employs two look-up tables to perform byte-basedcounting of bar/space transitions--one table is used when the previouspixel (i.e., the last bit in the previous byte) is a "1" and the othertable is used when the previous pixel is a "0." Each table has 256entries, one for every possible combination of pixels in an eight-bitbyte.

Each entry in each table represents three values: (a) the number of lead(left-most) pixels that are the same "color" (i.e., black or white) asthe previous pixel, (b) the number of trailing (right-most) pixels(including the last pixel) that are the same color as the last pixel inthe byte, and (c) the number of transitions within the eight-pixel byte(including any transition from the previous pixel).

For example, in the (previous pixel=1) table, the byte "01100110" has(a) "0" as the number of lead pixels that are the same color as theprevious pixel, (b) "1" as the number of trailing pixels that are thesame color as the last pixel, and (c) "5" as the number of transitionswithin the byte. In the (previous pixel=0) table, the byte "01100110"has (a) "1" as the number of lead pixels that are the same color as theprevious bit, (b) "1" as the number of trailing pixels that are the samecolor as the last pixel, and (c) "4" as the number of transitions withinthe byte.

Means 306 uses the values derived from the table entries to count thenumber of transitions and to determine the size of each bar and space.Means 306 keeps a running count of the number of transitions as itprocesses each sequence of image bytes along a selected search line.Means 306 also determines whether any bar or space exceeds specifiedthresholds by keeping a running count of the size of the currentbar/space. Means 306 performs these computations using the informationderived from the two tables.

For example, assume that a sequence of image bytes along a selectedsearch line is ("00011011", "01101101", "11001100", "00000111"). Assumefurther that the previous pixel for the first byte in the sequence was"0". After the first byte, the running count of the number oftransitions is 3. The second byte adds another 6 transitions to therunning count, for a total of 9. The third byte adds another 3transitions, for a total of 12. The fourth byte adds another 1transition, for a total of 13.

As each byte is processed, means 308 determines whether any bar or spaceis too wide by continuously incrementing and monitoring a currentbar/space size counter. For any given pixel along a selected searchline, this size counter represents the number of consecutive pixelssharing the same color as the given pixel and which also immediatelyprecede the given pixel along the search line. If a bar/space width isgreater than a specified maximum bar/space width (i.e., if the sizecounter exceeds a specified threshold), then the candidate symbol isrejected and means 308 directs processing to return to means 302 torestart byte-based searching for a candidate quiet zone. Otherwise,processing continues to means 310.

Referring again to the exemplary sequence of image bytes described twoparagraphs above, when the color changes from bright to dark in betweenthe end of the first byte and the beginning of the second byte, thecurrent bar/space size counter is reset to zero at this point along thesearch line. When the fourth byte is processed, the number of trailingpixels of the same color (2) from the third byte is increased by thenumber of leading pixels of the same color as the previous pixel (5) toyield a current bar/space size of 7 pixels.

If, in a particular application, the specified maximum bar/space widthis greater than five pixels, then the preceding discussion applies. If,however, the specified maximum bar/space width is five pixels or less,then special processing may be required. For example, if the maximum barwidth is five pixels, then the byte "10000001" would correspond to a barsix pixels wide. Those skilled in the art will understand that thissituation may be handled by manipulating the entries in the two tablesfor that byte. For example, if the numbers of leading pixels of the samecolor as the previous pixel stored in both tables for byte "10000001"were stored as six, then the processing described above would handlethis particular situation.

As each byte is processed, means 310 determines whether the runningcount of the number of transitions exceeds a specified threshold, forexample, 30. If so, then a candidate bar code symbol is recognized andprocessing continues to means 312; otherwise, processing returns tomeans 306 to continue the byte-based counting of transitions.

Processing reaches means 312 when a candidate bar code symbol isrecognized. Means 312 verifies the candidate symbol by repeating thebyte-based quiet-zone searching and transition counting of means 302through means 310 along selected neighboring search lines. Means 312preferably selects three parallel neighboring search lines on each sideof the original search line.

When the original search line is an image row, the neighboring searchlines are preferably every second or third row. For example, if row 40is the original search line, means 312 may select rows 31, 34, 37, 43,46, 49 as the six neighboring search lines.

Means 312 functions by first identifying the pixel along the originalsearch line that corresponds to the start of the candidate symbol. Thispixel is the first dark pixel following the candidate quiet zone. Basedon this first dark pixel, means 312 selects a starting column in thepixel image for each neighboring search line. Where searching isperformed along search lines from left to right, the starting column islocated left of the first dark pixel by a distance equivalent to thelength of at least two minimum quiet zones. For example, if the firstdark pixel along the original search line is at column 1000 and if theminimum size of a quiet zone is 32 pixels, then means 312 will beginsearching at column 936 in each neighboring search line.

Similarly, means 312 selects the ending column for each neighboringsearch line by moving from the first dark pixel to the right by adistance equivalent to the length of the maximum expected bar codesymbol plus two minimum quiet zones. Continuing with the previousexample, if the maximum symbol length is 300 pixels, then means 312 willend searching at column (1000+300+64) or 1364 of each neighboring searchline. In a preferred embodiment, a candidate bar code symbol is verifiedif at least three of the six neighboring search lines contain the samecandidate symbol. For a neighboring search line to contain the samecandidate symbol, the first black pixel in the neighboring search linemust be at a column within one-half of a minimum quiet zone from thecolumn that contained the first black pixel along the original searchline.

Those skilled in the art will understand that means 312 makes analogousselections when search lines are columns. It will also be understoodthat this preferred process of selecting neighboring search lines isdesigned to process images having bar code symbols of randomorientation. Those skilled in the art will recognize that byte-basedquiet-zone searching and transition counting can be implemented insystems having other than eight bits (i.e., pixels) per byte.

Referring again to FIG. 2, if a bar code symbol is located and verified,means 206 of symbol locator 104 directs processing to corner locator208; otherwise, processing continues to means 214.

The particular embodiment of byte-based searching for bar code symbolsin pixel images described in this section of this specification pertainsto the processing of only binary pixel images. Those skilled in the artwill understand that all other features of system 100 disclosed in thisspecification pertain to the processing of either binary or gray-scalepixel images.

Locating Corners of a Bar Code Symbol

Corner locator 208 of symbol locator 104 locates the four corners of abar code symbol located by means 204. Corner locator 208 receives frommeans 206 the coordinates of the first dark pixel along the originalsearch line and implements a corner-finding algorithm. In a preferredembodiment, the corner-finding algorithm implemented by locator 208 issimilar to that described in the section entitled "Locating the FourCorners of a Bar Code Symbol" and the three sections that immediatelyfollow that section in U.S. patent application Ser. No. 07/927,910, U.S.Pat. No. 5,343,028, the disclosure of which is incorporated herein inits entirety by reference.

Referring now to FIG. 4, there is shown an image of bar code symbol 400located by means 204, where pixel 402 is the first dark pixel along theoriginal search line. Briefly put, the corner-finding algorithmdescribed in U.S. patent application Ser. No. 07/927,910 comprises thefollowing steps:

(1) "Crawling" clockwise along outside edge 406 of leading bar 410 ofbar code symbol 400 from first dark pixel 402 until the end of bar 410is detected. The end of a bar is detected as a change in the crawlingdirection greater than a specified threshold. Such a change correspondsto crawling around a corner rather than along a line. This step locatesupper left-hand corner 408 of symbol 400;

(2) Crawling counter-clockwise along leading bar 410 to locate lowerleft-hand corner 426 of symbol 400;

(3) Projecting perpendicular line 412 from center pixel 404 of leadingbar 410 across symbol 400, as determined from corners 408 and 426;

(4) Searching along perpendicular line 412 from left to right startingat center pixel 404 for quiet zone 418 at the far end of symbol 400;

(5) Searching along perpendicular line 412 from right to left startingin quiet zone 418 for last dark pixel 420 at the far end of symbol 400;

(6) Crawling counter-clockwise along outer edge 422 of trailing bar 414to locate upper right-hand corner 416 of symbol 400; and

(7) Crawling clockwise along trailing bar 414 to locate lower right-handcorner 424 of symbol 400.

In addition to these seven steps of the corner-finding algorithmdisclosed in the '910 application, corner locator 208 preferablyverifies the accuracy of the located corners by performing the followingadditional steps, not disclosed in the '910 application:

(8) Verifying that the line segment defined by first dark pixel 402 andcorner 408 and the line segment defined by first dark pixel 402 andcorner 426 are sufficiently co-linear;

(9) Verifying that the line segment defined by last dark pixel 420 andcorner 416 and the line segment defined by last dark pixel 420 andcorner 424 are sufficiently co-linear; and

(10) Verifying that the line segment defined by corners 408 and 426 andthe line segment defined by corners 416 and 424 are sufficientlyparallel.

The candidate bar code symbol is rejected if the relevant line segmentsin steps (8), (9), or (10) are not sufficiently parallel. Two linesegments are sufficiently parallel if the difference between theirslopes is within a specified threshold. Since the relevant line segmentsof steps (8) and (9) share a common point (i.e., first dark pixel 402and last dark pixel 420, respectively), the test for parallelism isequivalent to the test for co-linearity. In a preferred embodiment,corner locator 208 determines the (row, column) coordinates in the pixelimage of the four located corners 408, 426, 416, and 424 of bar codesymbol 400.

"Squaring" the Located Symbol

After corner locator 208 locates four corners for the bar code symbol,means 210 corrects the location of at least one of the four corners. Thefour corners selected by corner locator 208 define a quadrilateral.However, a bar code symbol is ideally defined by a rectangle--aparticular type of quadrilateral. For one or more reasons, thequadrilateral defined by the four corners identified by corner locator208 may not be a rectangle.

For example, if the leading or trailing bar is broken into two or moresegments in the image, one side of the located quadrilateral may beshorter than the opposite side. Line noise in the transmission of arun-length-encoded facsimile may result in such breaks in the bars. Suchline noise may also result in extensions to the bars. In either case,the location of corners may be inaccurate. Alternatively, the actuallabel containing the imaged bar code symbol may be physically degradedsuch that the bars are not complete.

Referring now to FIG. 5, there is shown an image of a bar code symbol inwhich the trailing bar is broken into two segments. Corner locator 208locates corners A, B, C, and D having coordinates (Xa,Ya), (Xb,Yb),(Xc,Yc), and (Xd,Yd), respectively. Means 210 corrects the location ofat least one of the located corners by constructing the "best" rectanglebased on at least two of the other four located corners.

For each of the four corners, means 210 computes an error value E. Forexample, for corner B, the error value Eb is determined by: ##EQU1##where AC is the distance between corners A and C, AB is the distancebetween corners A and B, and BC is the distance between corners B and C.The greater the deviation from a ninety-degree right angle at corner B,the greater the error value Eb. Using similar equations, means 210 alsocalculates error values Ea, Ec, and Ed for corners A, C, and D,respectively.

Means 210 selects the corner with the greatest error as the corner tocorrect. For the symbol of FIG. 5, error value Eb is the greatest of thefour error values and means 210 selects corner B for correction. Thecorrected location for corner B may be defined by those coordinates(Xb',Yb') for which the error value of Equation (1) is zero. Typically,there are two unique solutions to Equation (1). Means 210 preferablyselects the solution closest to the originally located corner.

Those skilled in the art will understand that means 210 may be used tocorrect the location of at least two corners. Where, as in the previousexample, means 210 uses corners A and C to correct the location ofcorner B, means 210 may also use corners A and C to correct the locationof corner D, if desired, using an equation analogous to Equation (1). Ingeneral, means 210 may correct any two diagonal corners using theremaining two corners.

Those skilled in the art will understand that alternative methods forcorrecting the locations of one or more corners are within the scope ofthe present invention. These alternative methods may rely on one or moreof the following properties of rectangles (i.e., the ideal bar codesymbol shape):

Opposite sides of a rectangle are parallel;

Opposite sides are of equal length;

Diagonals are of equal length;

Corner angles are 90 degrees; and

Diagonals and their corresponding sides satisfy the Pythagorean theorem.

In addition, means 210 preferably performs geometry tests to verify thatthe size and shape of the squared bar code symbol are within specifiedthresholds. These geometry tests are based on the relative locations ofthe four symbol corners.

After means 210 squares the symbol, means 212 transmits the (row,column) coordinates of the four corners of the "corrected" symbol tocomposite signal generator 106 and gradient-based signal generator 112of FIG. 1. Processing of symbol locator 104 then continues to means 214to determine whether the end of the current search line has beenreached. If so, processing returns to means 202 to select a new searchline. Otherwise, processing returns to means 204 to continue the searchalong the current search line. Symbol locator 104 continues to attemptto locate bar code symbols until the specified search line sequence hasbeen exhausted or some other stop condition is satisfied.

Slow Method for Generating Composite Signals of Bar Code Symbols

Referring now to FIG. 6, there is shown a block flow diagram of a slowmethod implemented by composite signal generator 600. Composite signalgenerator 600 is a preferred embodiment of composite signal generator106 of system 100 depicted in FIG. 1, where generator 600 is designed toimplement the slow method for generating composite signals of bar codesymbols. According to a preferred embodiment of the slow method, the barcode symbol is scanned along multiple scan lines.

The scan lines are determined from the symbol corners determined bysymbol locator 104. For the slow method of generating composite signals,each scan line is parallel to the bars and spaces of the symbol. Inaddition, successive scan lines are separated from each other by adistance of less than one pixel width. When a bar code symbol is notaligned with the rows and columns of the pixel image, the scan lineswill not correspond to image rows or columns. Thus, scan lines for theslow method of generating composite signals generally differ from the"search lines" used by symbol locator 104, which always correspond toeither a row or column in the pixel image.

For each scan line, generator 600 averages the pixel intensities alongthe scan line to generate a pixel in a one-dimensional composite signal.By making the distance between successive scan lines less than onepixel, generator 600 oversamples the pixel image when generating thecomposite signal.

Referring now to FIG. 7, there is shown an image of bar code symbol 700processed by composite signal generator 600. Generator 600 receives thecoordinates of corners 702, 704, 706, and 708 from symbol locator 104.To avoid sampling errors common at the ends of symbol bars, generator600 identifies a sample rectangle 710 defined by corners 712, 714, 716,and 718. Sample rectangle 710 is defined to be slightly (e.g., 3%) lowerthan the top of symbol 700, slightly (e.g., 3%) higher than the bottomof symbol 700, and slightly longer than symbol 700 (e.g., by a specifieddistance into each quiet zone). The lengths of each scan line are thendefined by the boundaries of sample rectangle 710 and, as a result, eachscan line (which, by definition, is parallel to the bars and spaces ofsymbol 700) starts above the bottom and below the top of symbol 700.

Referring again to FIG. 6, means 602 of composite signal generator 600selects a sequence of parallel scan lines based on sample rectangle 710.Means 602 starts with the scan line defined by corners 712 and 718 atone end of sample rectangle 710 and selects each new scan line byshifting from the previous scan line toward the other end of samplerectangle 710 by a specified distance, where the specified distance isless than the width of a pixel.

Referring now to FIG. 8, a portion of bar code symbol 700 is shown withseveral scan lines 802 superimposed thereon. For each scan line 802,means 604 scans the symbol image along the currently selected scan lineand means 606 generates the average pixel intensity value of the pixelsthat lie along that scan line. Means 608 saves the average value as apixel in a one-dimensional composite signal. Means 610 determineswhether the current scan line is the last scan line, i.e., whether theopposite end of sample rectangle 710 defined by corners 714 and 716 hasbeen reached. If not, then processing returns to means 602 to select thenext scan line. Otherwise, means 612 transmits the complete compositesignal to composite signal thresholder 108 of system 100 for furtherprocessing. The processing of thresholder 108 is described later in thisspecification in conjunction with FIG. 11.

Fast Method for Generating Composite Signals of Bar Code Symbols

Referring now to FIG. 9, there is shown a block flow diagram of a fastmethod implemented by composite signal generator 900. Composite signalgenerator 900 is a preferred embodiment of composite signal generator106 of system 100 depicted in FIG. 1, where generator 900 is designed toimplement the fast method for generating composite signals of bar codesymbols. The fast method is a preferred alternative to the slow methoddescribed earlier in this specification in conjunction with FIGS. 6, 7,and 8.

According to a preferred embodiment of the fast method, generator 900selects a set of scan lines from the rows/columns in the pixel imagethat cross the bar code symbol. Note that, for the fast method, scanlines always correspond to either a row or column of the pixel image. Asdescribed earlier in this specification in conjunction with FIG. 6, forthe slow methods, scan lines need not and generally will not coincidewith a row or column of the pixel image.

For each scan line, generator 900 rotates and stretches the scan linedata. Using the stretched, rotated data, generator 900 updates aone-dimensional composite vector and a one-dimensional count vector.After all the selected scan lines have been processed, generator 900"normalizes" the one-dimensional vector to yield a one-dimensionalcomposite signal for the bar code symbol.

Referring now to FIG. 10, there is shown an image of the bar code symbolof FIG. 7 with scan lines 1-6 used by composite signal generator 900 toimplement the fast method of generating a composite signal superimposedthereon. Means 902 selects a sequence of scan lines. As shown in FIG.10, each scan line crosses the bar code symbol and is part of a row (orcolumn) of the pixel image. Means 902 need not select every row (orcolumn) that crosses the bar code symbol. Thus, successive scan linesmay be separated by more than one pixel. In addition, a scan line neednot cross the entire bar code symbol. For example, in FIG. 10, only scanlines 3 and 4 cross the entire symbol.

Means 904 rotates and stretches the data from each current scan line.The angle through which means 904 rotates the scan line data is dictatedby the orientation of the bar code symbol as indicated by the foursymbol corners identified by symbol locator 104. Those skilled in theart will understand that such rotation effectively aligns the bars ofthe bar code symbol with the columns (or rows) of a rotated image.

The degree to which means 904 stretches the scan line data is determinedby the oversampling rate, a specified parameter value, for example,three. Stretching the data in the fast method is similar to theselection of scan lines in the slow method that are separated bysubpixel distances. Both the fast and slow methods are designed tooversample the original pixel image data.

Tables II through VI present an example of the processing performed bycomposite signal generator 900. The purpose of the discussion of thesetables is to demonstrate the processing implemented by generator 900 andis not necessarily intended to represent a realistic situation. Notethat lists (I), (J), (K), and (L) of Tables III, IV, and V will bediscussed later in this specification in conjunction with the discussionof FIG. 15 and gradient signal generator 112.

Table II contains the original pixel intensity data for three scan linesof a gray-scale pixel image, where scan line #1 corresponds to row 0 ofthe image, scan line #2 corresponds to row 2, and scan line #3corresponds to row 4. In this example, the higher the pixel intensityvalue, the brighter the pixel. Each scan line of Table II begins atcolumn 0 and ends at column 9 of the pixel image. Thus, for example, theintensity of the pixel at (row=2) and (column=5) is 3. In this example,it has been determined, based upon the four corners located by symbollocator 104, that the bar code symbol is oriented at a counter-clockwise45-degree angle to the image rows.

                  TABLE II                                                        ______________________________________                                        Original Image Data.                                                                 Scan Line                                                                       #1            #2     #3                                                       Row           Row    Row                                             Col      0             2      4                                               ______________________________________                                        0        21            22     22                                              1        23            21     19                                              2        22            22     5                                               3        21            5      4                                               4        22            6      3                                               5        18            3      15                                              6        6             16     18                                              7        4             18     17                                              8        5             16     20                                              9        14            18     5                                               ______________________________________                                    

Table III presents the results from generator 900 processing scan line#1 (row 0). List (A) of Table III contains the original column numberfor each pixel in the pixel image along scan line #1. List (B) containsthe intensity of each original pixel. List (C) contains the rotatedcolumn number (a real value) after means 904 rotates each original pixelby θ degrees, where, in this example, θ is 45 degrees. The rotatedcolumn number C_(r) is determined by:

    C.sub.r =C.sub.o cosθ+R.sub.o sinθ,            (2)

where C_(o) is the original column number and R_(o) is the original rownumber for the pixel in the pixel image.

                                      TABLE III                                   __________________________________________________________________________    Processing scan Line #1 (Row 0).                                                    (C)                                                                              (D) (E)   (G)                                                                              (H)                                                                              (I)                                                                              (J)                                                                              (K)                                                                              (L)                                         (A)                                                                              (B)                                                                              Rot                                                                              Str Rnd                                                                              (F)                                                                              Cmp                                                                              Ct Wht                                                                              Wht                                                                              Blk                                                                              Blk                                         Col                                                                              Int                                                                              Col                                                                              Col Col                                                                              Int                                                                              Vec                                                                              Vec                                                                              Grad                                                                             Vec                                                                              Grad                                                                             Vec                                         __________________________________________________________________________    0  21 0.00                                                                             0.00                                                                              0  21 21 1  0  0  0  0                                                        1  21 21 1  0  0  0  0                                           1  23 0.71                                                                             2.12                                                                              2  23 23 1  2  2  0  0                                                        3  23 23 1  2  2  0  0                                           2  22 1.41                                                                             4.24                                                                              4  22 22 1  0  0  1  1                                                        5  22 22 1  0  0  1  1                                           3  21 2.12                                                                             6.36                                                                              6  21 21 1  0  0  1  1                                                        7  21 21 1  0  0  1  1                                           4  22 2.83                                                                             8.49                                                                              8  22 22 1  1  1  0  0                                                        9  22 22 1  1  1  0  0                                                        10 22 22 1  1  1  0  0                                           5  18 3.54                                                                             10.61                                                                             11 18 18 1  0  0  4  4                                                        12 18 18 1  0  0  4  4                                           6  6  4.24                                                                             12.73                                                                             13 6  6  1  0  0  12 12                                                       14 6  6  1  0  0  12 12                                          7  4  4.95                                                                             14.85                                                                             15 4  4  1  0  0  2  2                                                        16 4  4  1  0  0  2  2                                           8  5  5.66                                                                             16.97                                                                             17 5  5  1  1  1  0  0                                                        18 5  5  1  1  1  0  0                                           9  14 6.36                                                                             19.09                                                                             19 14 14 1  9  9  0  0                                                        20 14 14 1  9  9  0  0                                           __________________________________________________________________________     KEY                                                                           (A): Column of pixel in original image                                        (B): Intensity of pixel in original image                                     (C): Column (real value) of pixel after rotation                              (D): Column (real value) of pixel after rotation and stretching               (E): Column (rounded value) of rotated, stretched pixel                       (F): Intensity of rotated, stretched pixel                                    (G): Corresponding value in composite vector                                  (H): Corresponding value in count vector                                      (I): Whitening gradient for current scan line                                 (J): Corresponding value in whitening gradient vector                         (K): Blackening gradient for current scan line                                (L): Corresponding value in blackening gradient vector                   

List (D) of Table III contains the stretched column number (a realvalue) after means 904 stretches each rotated pixel by the oversamplingrate. This example is based on an oversampling rate of three. List (D)is the product of list (C) and the oversampling rate (three, in thisexample). List (E) contains the rounded column numbers (integer values)for the stretched and rotated data, as generated by means 904. Forexample, stretched column 8.49 from list (D) is rounded to roundedcolumn 8 in list (E). Similarly, stretched column 10.61 from list (D) isrounded to rounded column 11 in list (E). List (E) also contains thecolumn numbers falling between the rounded columns. Thus, column 7 fallsbetween rounded columns 6 and 8, while columns 9 and 10 fall betweenrounded columns 8 and 11. For purposes of the following discussion, allof the elements of list (E) are collectively referred to as roundedcolumns.

List (F) contains an intensity value for each rounded column in list(E). For each rounded column of list (E), means 904 repeats theintensity value from the original column of list (A) correspondingthereto. For example, rounded columns 6 and 7 of list (E) correspond tocolumn 3 of list (A) and, therefore, the corresponding intensities oflist (F) for those rounded columns are 21.

In an alternative preferred embodiment, means 904 interpolates betweenrounded columns to determine the intensity values in list (F). In thatembodiment, the intensity in list (F) corresponding to rounded column 7of list (E) would be 21.5, the midpoint of the intensities correspondingto rounded columns 6 and 8 of list (E).

Those skilled in the art will understand that the rotating andstretching implemented by means 904 may be performed in a singlecomputation based on matrix multiplication where the rotation matrix ismodified to include the oversampling rate.

As means 904 rotates and stretches the data for each scan line, means906 updates a one-dimensional composite vector and means 908 updates aone-dimensional count vector based on the stretched, rotated data. Thecomposite vector represents the "sum" of all the previously stretched,rotated scan lines. The count vector keeps track of how many times eachelement of the composite vector has been updated. Referring again to theexample of Table III, list (G) represents the composite vector and list(H) represents the count vector after scan line #1 is processed.

After means 908 updates the count vector, means determines whether thecurrent scan line is the last scan line to be processed. If so,processing continues to means 912; otherwise, processing returns tomeans 902 to select the next scan line. Since, in the example of TableII, row 0 (i.e., scan line #1) is not the last scan line, means 910directs processing to return to means 902 to select row 2 as the newscan line.

Table IV presents the results from generator 900 processing scan line #2(row 2) of Table II. Lists (A) through (H) of Table IV are analogous tothose of Table III. Means 906 forms an updated composite vector (list(G) of Table IV) by summing pixel values from the prior composite vector(list (G) of Table III) with corresponding intensity values from list(F) of Table IV. Means 908 updates the count vector of list (H) byincrementing those elements that correspond to elements of the compositevector of list (G) that were updated.

                                      TABLE IV                                    __________________________________________________________________________    Processing scan Line #2 (Row 2).                                                    (C)                                                                              (D) (E)   (G)                                                                              (H)                                                                              (I)                                                                              (J)                                                                              (K)                                                                              (L)                                         (A)                                                                              (B)                                                                              Rot                                                                              Str Rnd                                                                              (F)                                                                              Cmp                                                                              Ct Wht                                                                              Wht                                                                              Blk                                                                              Blk                                         Col                                                                              Int                                                                              Col                                                                              Col Col                                                                              Int                                                                              Vec                                                                              Vec                                                                              Grad                                                                             Vec                                                                              Grad                                                                             Vec                                         __________________________________________________________________________                 0     21 1     0     0                                                        1     21 1     0     0                                                        2     23 1     2     0                                                        3     23 1     2     0                                           0  22 1.41                                                                             4.24                                                                              4  22 44 2  0  0  0  1                                                        5  22 44 2  0  0  0  1                                           1  21 2.12                                                                             6.36                                                                              6  21 42 2  0  0  1  2                                                        7  21 42 2  0  0  1  2                                           2  22 2.83                                                                             8.49                                                                              8  22 44 2  1  2  0  0                                                        9  22 44 2  1  2  0  0                                                        10 22 44 2  1  2  0  0                                           3  5  3.54                                                                             10.61                                                                             11 5  23 2  0  0  17 21                                                       12 5  23 2  0  0  17 21                                          4  6  4.24                                                                             12.73                                                                             13 6  12 2  1  1  0  12                                                       14 6  12 2  1  1  0  12                                          5  3  4.95                                                                             14.85                                                                             15 3  7  2  0  0  3  5                                                        16 3  7  2  0  0  3  5                                           6  16 5.66                                                                             16.97                                                                             17 16 21 2  13 14 0  0                                                        18 16 21 2  13 14 0  0                                           7  18 6.36                                                                             19.09                                                                             19 18 32 2  2  11 0  0                                                        20 18 32 2  2  11 0  0                                           8  16 7.07                                                                             21.21                                                                             21 16 16 1  0  0  2  2                                                        22 16 16 1  0  0  2  2                                           9  18 7.78                                                                             23.33                                                                             23 18 18 1  2  2  0  0                                                        24 18 18 1  2  2  0  0                                           __________________________________________________________________________

Note that none of the stretched, rotated data from scan line #2coincides with rounded columns 0-3, but that some of the data docoincide with new rounded columns 21-24. As a result, some elements ofthe composite vector are not updated. The count vector is used to keeptrack of how many times each element of the composite vector is updated.Those skilled in the art will understand that this non-uniform updatingof composite and count vectors will also occur when processing scanlines that do not cross the entire bar code symbol, for example, scanline 1 of FIG. 10.

After processing scan line #2, means 910 directs processing to return tomeans 902 to select scan line #3 (row 4), the last scan line of TableII. Table V presents the results from generator 900 processing scan line#3 (row 4) of Table II. Once again, lists (A) through (H) of Table V areanalogous to those of Tables III and IV.

                                      TABLE V                                     __________________________________________________________________________    Processing Scan Line #3 (Row 4).                                                    (C)                                                                              (D) (E)   (G)                                                                              (H)                                                                              (I)                                                                              (J)                                                                              (K)                                                                              (L)                                         (A)                                                                              (B)                                                                              Rot                                                                              Str Rnd                                                                              (F)                                                                              Cmp                                                                              Ct Wht                                                                              Wht                                                                              Blk                                                                              Blk                                         Col                                                                              Int                                                                              Col                                                                              Col Col                                                                              Int                                                                              Vec                                                                              Vec                                                                              Grad                                                                             Vec                                                                              Grad                                                                             Vec                                         __________________________________________________________________________                 0     21 1     0     0                                                        1     21 1     0     0                                                        2     23 1     2     0                                                        3     23 1     2     0                                                        4     44 2     0     1                                                        5     44 2     0     1                                                        6     42 2     0     2                                                        7     42 2     0     2                                           0  22 2.83                                                                             8.49                                                                              8  22 66 3  0  2  0  0                                                        9  22 66 3  0  2  0  0                                                        10 22 66 3  0  2  0  0                                           1  19 3.54                                                                             10.61                                                                             11 19 42 3  0  0  3  24                                                       12 19 42 3  0  0  3  24                                          2  5  4.24                                                                             12.73                                                                             13 5  17 3  0  1  14 36                                                       14 5  17 3  0  1  14 36                                          3  4  4.95                                                                             14.85                                                                             15 4  11 3  0  0  1  6                                                        16 4  11 3  0  0  1  6                                           4  3  5.66                                                                             16.97                                                                             17 3  24 3  0  14 1  1                                                        18 3  24 3  0  14 1  1                                           5  15 6.36                                                                             19.09                                                                             19 15 47 3  12 23 0  0                                                        20 15 47 3  12 23 0  0                                           6  18 7.07                                                                             21.21                                                                             21 18 34 2  3  3  0  2                                                        22 18 34 2  3  3  0  2                                           7  17 7.78                                                                             23.33                                                                             23 17 35 2  0  2  1  1                                                        24 17 35 2  0  2  1  1                                           8  20 8.49                                                                             25.46                                                                             25 20 20 1  3  3  0  0                                                        26 20 20 1  3  3  0  0                                                        27 20 20 1  3  3  0  0                                           9  5  9.19                                                                             27.58                                                                             28 5  5  1  0  0  15 15                                                       29 5  5  1  0  0  15 15                                          __________________________________________________________________________

After processing scan line #3, means 910 directs processing to continueto means 912. Means 912 "normalizes" the composite vector to generate aone-dimensional composite signal by dividing each element in thecomposite vector by the corresponding element in the count vector. Means914 then transmits this composite signal to composite signal thresholder108 of system 100.

Table VI presents the one-dimensional composite signal generated bymeans 912. Lists (E), (G), and (H) are identical to those lists of TableV. List (M) represents the composite signal generated by means 912 bydividing the elements of the composite vector of list (G) by thecorresponding elements of the count vector of list (H). Means 914transmits the composite signal of list (M) to thresholder 108.

                  TABLE VI                                                        ______________________________________                                        Processing Composite Signal.                                                  (E)       (G)    (H)         (M)  (N)                                         Rnd       Cmp    Ct          Cmp  Thr                                         Col       Vec    Vec         Sig  Sig                                         ______________________________________                                        0         21     1           21.0 1                                           1         21     1           21.0 1                                           2         23     1           23.0 1                                           3         23     1           23.0 1                                           4         44     2           22.0 1                                           5         44     2           22.0 1                                           6         42     2           21.0 1                                           7         42     2           21.0 1                                           8         66     3           22.0 1                                           9         66     3           22.0 1                                           10        66     3           22.0 1                                           11        42     3           14.0 1                                           12        42     3           14.0 1                                           13        17     3           5.7  0                                           14        17     3           5.7  0                                           15        11     3           3.7  0                                           16        11     3           3.7  0                                           17        24     3           8.0  0                                           18        24     3           8.0  0                                           19        47     3           15.7 1                                           20        47     3           15.7 1                                           21        34     2           17.0 1                                           22        34     2           17.0 1                                           23        35     2           17.5 1                                           24        35     2           17.5 1                                           25        20     1           20.0 1                                           26        20     1           20.0 1                                           27        20     1           20.0 1                                           28        5      1           5.0  0                                           29        5      1           5.0  0                                           ______________________________________                                         KEY                                                                           (M): Normalized composite signal = composite vector/count vector              (N): Thresholded composite signal (threshold value = 12.0)               

Thresholding and Filtering the One-Dimensional Composite Signal

Referring now to FIG. 11, there is shown a block flow diagram of theprocessing implemented by composite signal thresholder 108 of system 100to threshold a one-dimensional composite signal of a bar code symbol.Thresholder 108 processes a composite signal, whether the compositesignal was generated by generator 600 using the slow method describedearlier in this specification in conjunction with FIG. 6 or by generator900 using the fast method described earlier in this specification inconjunction with FIG. 9. Thresholder 108 thresholds the composite signalinto binary black ("0") and white ("1") segments that correspond to thebars and spaces of the bar code symbol. Thresholder 108 then filters thethresholded signal to eliminate spurious bars and spaces. Thresholder108 then transmits the filtered signal to composite signal decoder 110,which uses conventional bar code symbol decoding methods to decode thefiltered signal.

Means 1102 of thresholder 108 thresholds the composite signal receivedfrom generator 106. Means 1102 preferably employs adaptive thresholdingto threshold the composite signal, although means 1102 may use anyconventional thresholding algorithm including thresholding with a fixedthreshold value.

Referring again to Table VI, list (N) represents the binary signalgenerated by means 1102 by thresholding the composite signal of list (M)using a fixed threshold value of 12. If an element of the compositesignal is greater than 12, then the corresponding binary element is 1(white); otherwise the corresponding binary element is 0 (black). In theexample of Tables II through VI, the fast method of generating acomposite signal indicates a bright-to-dark transition between roundedcolumns 12 and 13, a dark-to-bright transition between rounded columns18 and 19, and another bright-to-dark transition between rounded columns27 and 28. These transitions correspond to the edges of bars in the barcode symbol.

Means 1102 preferably employs an adaptive thresholding algorithm tolocate transitions in the composite signal. Under a preferred adaptivethresholding algorithm, means 1102 uses the local minimum and maximumintensity values in the composite signal to determine the thresholdvalue used to locate the transition between those local minima andmaxima.

Referring again to Table VI, the first local maximum intensity value is23.0 corresponding to rounded columns 2 and 3 and the first localminimum intensity value is 3.7 corresponding to rounded columns 15 and16. In this example, means 1102 selects one-half the sum of the localmaximum and minimum (0.5×(23.0+3.7)) or 13.4 as the threshold value tobe applied between rounded column 2 and rounded column 16. Using thatthreshold value, means 1102 locates the bright-to-dark transitionbetween rounded columns 12 and 13. If, in a particular situation, two ormore consecutive elements in the composite signal are equal to thethreshold value, the middle element of that series of consecutiveelements is selected as the bright/dark transition.

To minimize the effect of noise, local maxima and minima are defined interms of a specified significance value. If the difference between alocal maximum and the next local minimum is not greater than thespecified significance value, then means 1102 rejects them and continuesto search for proper local minimum and maximum pixels in the compositesignal.

For example, consider the composite signal having intensity values (5,3, 6, 25, 22, 26, 10, 4, 7). Assume that the significance value isdetermined to be 5. Means 1102 scans from left to right to find a localminimum at 3. Means 1102 continues to scan to find a local maximum at25. Since the difference between the local maximum (25) and the localminimum (3) is greater than the significance value (5), means 1102retains these values.

Means 1102 then scans for the next local minimum at 22. Since thedifference between the local maximum (25) and the local minimum (22) isnot greater than the significance value (5), means 1102 continues tolook for a proper local minimum. While looking for a proper localminimum, means 1102 updates the local maximum, if necessary. Thus, asmeans 1102 scans to find the next local minimum at 4, means 1102 updatesthe local maximum to be 26. Since the difference between the updatedlocal maximum (26) and the new local minimum (4) is greater than thesignificance value (5), means 1102 selects these pixels as the properlocal maximum and minimum. These local minimum and maximum values arethen averaged to calculate the threshold value used to find the pixelcorresponding to the transition between the local maximum and minimum.Note that if the significance value had been 2 instead of 5, then 25 and26 would both be considered to be local maxima.

The selected significance value determines the immunity of means 1102 tonoise. High significance values provide substantial noise immunity, butmay also result in the loss of true bright/dark transitions in thecomposite signal. Low significance values, on the other hand, may leadto the spurious recognition of transitions in noisy regions. It ispreferable to use significance values that are too low rather than toohigh. This is true because subsequent filtering may be able to eliminatespurious transitions, but nothing can be done to recover from the lossof true transitions. In a preferred embodiment, the significance valueis based on a fraction (e.g., one-eighth) of the dynamic range of thecomposite signal as defined by the overall minimum and maximum valueswithin the composite signal.

Referring now to FIGS. 12 and 13, there are shown two common examples ofspurious bright/dark transitions that may result from using relativelylow significance values to threshold a noisy composite signal. FIG. 12depicts the binary representation of a spurious bar 1 in what should bea single space. FIG. 13 depicts the binary representation of either aspurious bar 1 or a spurious space 2, where only one space and one barshould exist.

Means 1104 filters the thresholded composite signal to remove spuriousbars and spaces. A spurious bar/space is one that is too narrow to bevalid as determined by the known characteristics of the bar codesymbology being decoded. Threshold values may be set based on certainfractions (e.g., one-half) of the minimum bar and space widths in theparticular symbology to be decoded. If a bar or space is narrower thanthe specified threshold, then the bar/space is removed. In FIG. 12,means 1104 removes the spurious bar 1 to create a single space, becausebar 1 is too narrow to be valid.

In FIG. 13, bar 1 and space 2 may both be narrower than the applicablethresholds. In such case, means 1104 eliminates the smaller of the twoby merging the neighboring bars/spaces together. Thus, if bar 1 isnarrower than space 2, then means 1104 removes bar 1 to yield a singlespace that ends at point 4. If, however, bar 1 is not narrower thanspace 2, then means 1104 removes space 2 to yield a single space thatends at point 3.

Referring again to FIG. 11, after means 1104 filters the thresholdedsignal, means 1106 transmits the filtered signal to composite signaldecoder 110, which decodes the filtered signal using conventional barcode symbol decoding methods.

Decoding Composite Signals

Composite signal decoder 110 decodes the filtered composite signalsgenerated by thresholder 108 using conventional bar code symbol decodingmethods. Decoder 110 may be designed to decode any desired symbology,such as Code 128, Code 39, and Interleaved Code 2 of 5.

When decoding Code 128 bar code symbols, decoder 110 makes standardlike-edge to like-edge measurements as described in the section entitled"Determining Character Choices from Subpixel Interpolation Results" ofU.S. patent application Ser. No. 07/927,910, the disclosure of which isincorporated herein by reference. Decoder 110 calculates the standardt1, t2, t3, and t4 values, scans a mapping table, and performs Code 128checksum computations to support decoding of all three character sets ofCode 128.

Referring now to FIG. 14, there is shown a block flow diagram of theprocessing implemented by composite signal decoder 110 of system 100 todecode the characters of a Code 39 bar code symbol. Code 39 symbology isbased on characters having five bars and four spaces. In Code 39, threeof the bars/spaces must be wide and the other six bars/spaces must benarrow. In addition, a Code 39 character must have either two wide barsand one wide space, or three wide spaces. A character with exactly zero,two, or four wide spaces is not a valid Code 39 character. Similarly, ifa character has exactly one wide space, but does not have exactly twowide bars, then it is not a valid Code 39 character. When decoding Code39 bar code symbols, decoder 110 examines five bars and four spaces ofthe bar code symbol at a time, classifies them as wide or narrow, anddecodes the character by referring to a look-up table.

To decode a Code 39 character, decoder 110 first analyzes the fourspaces of the character. Means 1402 in FIG. 14 calculates a space-widththreshold for the character, where the space-width threshold is theaverage of the width of the widest space and the width of the narrowestspace in the character.

Means 1404 then classifies each of the four spaces as either wide ornarrow by comparing the width of each space to the space-widththreshold. Means 1404 classifies a space as wide if its width is greaterthan the space-width threshold; otherwise, the space is narrow.

If means 1404 classifies three of the spaces as wide and one as narrow,then means 1406 directs processing to means 1416 to decode thecharacter; otherwise, processing continues to means 1408. If decoder 110determines that there are three wide spaces and one narrow space, thendecoder 110 assumes that all five bars are narrow and that the characteris a valid Code 39 character.

If means 1404 classifies one of the spaces as wide and three as narrow,then means 1408 directs processing to means 1410; otherwise, thecharacter cannot be decoded. If decoder 110 determines that there arenot exactly one or three wide spaces, then the character is not a validCode 39 character and cannot be decoded as such.

If decoder 110 determines that there are three narrow spaces and onewide space, then decoder 110 analyzes the five bars of the character.Means 1410 calculates a bar-width threshold for the character, where thebar-width threshold is the average of the width of the widest bar andthe width of the narrowest bar in the character.

Means 1412 then classifies each of the five bars as either wide ornarrow by comparing the width of each bar to the bar-width threshold.Means 1412 classifies a bar as wide if its width is greater than thebar-width threshold; otherwise, the bar is narrow.

If means 1412 classifies two of the bars as wide and three as narrow,then means 1414 directs processing to means 1416 to decode thecharacter; otherwise, the character is not a valid 1416 Code 39character and cannot be decoded as such. If, after determining thatthere is exactly one wide space, decoder 110 determines that there aretwo wide bars and three narrow bars, then decoder 110 assumes that thecharacter is a valid Code 39 character.

After successfully locating the three wide bars/spaces and six narrowbars/spaces of the Code 39 character, means 1416 decodes the characterby searching through a Code 39 look-up table for the appropriatealphanumeric character using the sequence of wide and narrow bars andspaces identified by means 1404 and 1412. In a preferred embodiment,decoder 110 also performs Code 39 checksum analysis.

In the preferred embodiment of FIG. 14, decoder 110 analyzes the widthsof bars and spaces independently by calculating two different widththresholds--one for classifying spaces and one for classifying bars.Decoder 110 uses these different width thresholds for spaces and bars toreduce decoding errors that may result from pixel image bleeding.

When pixel images are generated using conventional imaging systems, darkregions in the images often tend to bleed into bright regions. As aresult of this bleeding, the bars in imaged bar code symbols may appearwider and spaces may appear narrower than those in the true bar codesymbol. At times, true narrow bars (that is, a bar that is narrow in thetrue bar code symbol) may even appear in the pixel image to be widerthan true wide spaces. These apparent changes in the widths of bars andspaces may lead to errors in the decoding of the bar code symbols. Whendecoding Code 39 symbols, for example, decoder preferably classifiesbars and spaces independently using different width thresholds to reducethe decoding errors that result from pixel image bleeding.

Those skilled in the art will understand that this independentclassification of bars and spaces in bar code symbols to correct for theeffects of bleeding may be employed to decode symbols other than thoseof the Code 39 symbology.

For example, decoder 110 preferably uses independent classification ofbars and spaces to decode Interleaved Code 2 of 5 (I2of5) bar codesymbols. In I2of5 characters, five bars and five spaces encode twodecimal digits. The bars encode the first digit while the spaces encodethe second digit. Each digit is represented by two wide bars (or spaces)and three narrow bars (or spaces). Decoder 110 takes thirteen bars andspaces of an I2of5 symbol at a time and classifies them as wide ornarrow by independently classifying the bars and spaces, similar to theindependent classification scheme implemented by decoder 110 for Code 39symbols. The result is a 13-bit value where each bit represents thewidth of a corresponding bar/space.

Decoder 110 then searches an I2of5 look-up table twice, once for thebars and once for the spaces. Decoder 110 uses only the first tenbars/spaces (top 10 bits) during the table look-up. The other three bitsmay correspond to the I2of5 stop pattern. If the last three bits are avalid stop pattern (wide bar, narrow space, wide bar), then decodingstops. Otherwise, another thirteen bars and spaces are analyzed, wherethe first three bars and spaces are the last three bars and spaces fromthe previous list. Decoder 110 also preferably performs I2of5 checksumanalysis.

After decoding the thresholded composite signal, decoder 110 transmitsthe decoded signal to output selector 118 of system 100.

Generating Gradient Signals for a Bar Code Symbol

Referring now to FIG. 15, there is shown a block flow diagram of theprocessing implemented by gradient signal generator 112 of system 100 togenerate whitening and blackening gradient signals for a bar code symbollocated by symbol locator 104. The gradient signals are used to find thetransitions between bars and spaces (i.e., leading and trailing edges ofbars) in the bar code symbol.

In a preferred embodiment, generator 112 selects scan lines from the setof rows or columns in the pixel image that cross the bar code symbol.These scan lines are preferably the same scan lines as those selectedfor the fast method for generating composite signals, described earlierin this specification in conjunction with FIG. 9.

For each scan line, generator 112 rotates and stretches the scan linedata and updates two one-dimensional gradient vectors--a whiteninggradient vector and a blackening gradient vector--and a one-dimensionalcount vector based on the stretched, rotated data. After all theselected scan lines have been processed, generator 112 normalizes thetwo gradient vectors to generate whitening and blackening gradientsignals, smoothes the gradient signals, and transmits the smoothedgradient signals to gradient signal processor 114 for furtherprocessing.

Means 1502 selects a sequence of scan lines. Means 1502 preferablyselects the same sequence of scan lines as that selected by means 902 ofcomposite signal generator 900 in FIG. 9. Means 1504 rotates andstretches the data from the currently selected scan line. Means 1504preferably performs the same rotation and stretching function as thatperformed by means 904 of generator 900. In a preferred embodiment,operations common to composite signal generator 900 and gradient signalgenerator 112 (e.g., the selection, rotation, and stretching of scanlines) are combined to avoid duplication.

Referring again to the example of Table II, the processing of means 1504for each of the scan lines is identical to the processing performed bymeans 1504, as described earlier in this specification in conjunctionwith FIG. 9. Thus, lists (A) through (F) of Tables III, IV, and V mayalso be used describe the operation of gradient signal generator 112.

After means 1504 rotates and stretches the first scan line, means 1506updates a whitening and a blackening gradient vector using the rotated,stretched data. In a preferred embodiment, the whitening gradient vectoris a one-dimensional vector that accumulates the magnitudes of all thepixel-to-pixel changes in which the pixel intensity values increase.Analogously, the blackening gradient vector is a one-dimensional vectorthat accumulates the magnitudes of all the pixel-to-pixel changes inwhich the pixel intensity values decrease. For each gradient vector, themagnitude of the difference between adjacent image pixels isaccumulated, not the absolute pixel values themselves. In an alternativepreferred embodiment, the whitening and blackening gradient vectorsaccumulate the number of pixel-to-pixel changes that increase anddecrease, respectively, as opposed to accumulating the magnitudes ofthose changes.

The whitening and blackening gradients may be defined in terms of thefirst derivative of the scan line data. The whitening gradientscorrespond to those first derivatives that are greater than zero, whilethe blackening gradients correspond to the magnitudes of the firstderivatives that are less than zero.

Referring again to Table III, list (I) represents the whiteninggradients for scan line #1 and list (J) represents the whiteninggradient vector after means 1506 processes scan line #1. In general, thewhitening gradient vector represents the sum of corresponding whiteninggradients for all the scan lines that have been processed. Since scanline #1 is the first scan line to be processed in the example, thewhitening gradients of list (I) are identical to the whitening gradientvector of list (J).

Since the intensity increases from 21 to 23 from pixel (0,0) to pixel(0,1) in the original data, the corresponding whitening gradients inlist (I) for rounded columns 2 and 3 are 2. If the change from theprevious pixel to the current pixel in the original image is notpositive, then there is no whitening gradient for that current pixel.For example, since the intensity decreases from 23 to 22 from pixel(0,1) to pixel (0,2) in the original data, the corresponding whiteninggradients in list (I) for rounded columns 4 and 5 are 0.

Similarly, list (K) of Table III represents the blackening gradients forscan line #1. For example, since the intensity decreases from 22 to 18from pixel (0,4) to pixel (0,5) in the original data, the correspondingblackening gradients for rounded columns 11 and 12 are 4. If the changefrom the previous pixel to the current pixel in the original image isnot negative, then there is no blackening gradient for that currentpixel. For example, since the intensity increases from 4 to 5 from pixel(0,7) to pixel (0,8) in the original data, the corresponding blackeninggradients for rounded columns 17 and 18 are 0. List (L) of Table IIIrepresents the blackening gradient vector (i.e., the accumulatedblackening gradients) after means 1506 processes scan line #1.

Means 1506 also updates a one-dimensional count vector that is identicalto the count vector generated by means 908 of composite signal generator900, as described earlier in this specification in conjunction with FIG.9. Thus, list (H) of Tables III, IV, and V also applies to thedescription of gradient signal generator 112.

After means 1506 updates both gradient vectors and the count vector forthe current scan line, means 1508 determines whether the current scanline is the last scan line for the bar code symbol. If not, thenprocessing returns to means 1502 to select the next scan line.Otherwise, processing proceeds to means 1510.

In the example of Table II, after scan line #1 is processed, processingreturns to means 1502 to select scan line #2 as the new scan line. TableIV presents the results of processing scan line #2. Lists (I) and (K) ofTable IV represent the whitening and blackening gradients, respectively,for scan line #2. Lists (J) and (L) represent the updated gradientvectors. Means 1506 updates the gradient vectors by "adding" thegradients for scan line #2 to the gradient vectors of Table III.Similarly, Table V presents the results of processing scan line #3.

After the last scan line is processed, means 1508 directs processing tomeans 1510. Means 1510 normalizes the whitening and blackening gradientvectors by dividing the elements of each gradient vector by thecorresponding elements of the count vector.

Tables VII and VIII present the results of processing the whitening andblackening gradient vectors, respectively. Lists (E), (H), and (J) ofTable VII and lists (E), (H), and (L) of Table VIII are identical to thecorresponding lists of Table V. List (O) of Table VII represents thewhitening gradient signal that results from means 1510 normalizing thewhitening gradient vector of list (J). Means 1510 determines eachelement in list (O) by dividing the corresponding element of list (J) bythe corresponding element of list (H). Similarly, list (T) of Table VIIIrepresents the blackening gradient signal that results from means 1510normalizing the blackening gradient vector of list (L).

                  TABLE VII                                                       ______________________________________                                        Processing Whitening Gradient.                                                (E)   (H)    (J)     (O)   (P)   (Q)   (R)   (S)                              Rnd   Ct     Wht     Wht   Smth  Seg-  Seg   WhSeg                            Col   Vec    Vec     Sig   WSig  #     Mass  Cntrd                            ______________________________________                                        0     1      0       0.0   0.0   --        0                                  1     1      0       0.0   0.5             0                                  2     1      2       2.0   1.5   1     4.0    0*                              3     1      2       2.0   1.5               0                                4     2      0       0.0   0.5               0                                5     2      0       0.0   0.0   --        0                                  6     2      0       0.0   0.0             0                                  7     2      0       0.0   0.2             0                                  8     3      2       0.7   0.5             0                                  9     3      2       0.7   0.7   2     2.2    0*                              10    3      2       0.7   0.5               0                                11    3      0       0.0   0.2               0                                12    3      0       0.0   0.1   --        0                                  13    3      1       0.3   0.2   3     0.6    0*                              14    3      1       0.3   0.2               0                                15    3      0       0.0   0.1   --        0                                  16    3      0       0.0   1.2             0                                  17    3      14      4.7   3.5             0                                  18    3      14      4.7   5.5             0                                  19    3      23      7.7   7.0   4     29.1  1                                20    3      23      7.7   6.2               0                                21    2      3       1.5   3.1               0                                22    2      3       1.5   1.4               0                                23    2      2       1.0   1.1   --        0                                  24    2      2       1.0   1.5             0                                  25    1      3       3.0   2.5             0                                  26    1      3       3.0   3.0   5     11.2   0*                              27    1      3       3.0   2.3               0                                28    1      0       0.0   0.8               0                                29    1      0       0.0   0.0   --        0                                  ______________________________________                                         KEY                                                                           (O): Normalized whitening gradient signal = whitening gradient vector /       count vector                                                                  (P): Smoothed whitening gradient signal = (previous pixel + (2 ×        pixel) + next pixel)/4                                                        (Q): Segment number                                                           (R): Segment mass                                                             (S): Weighted centroid of whitening gradient segment                          *segment rejected because of low mass                                    

                  TABLE VIII                                                      ______________________________________                                        Processing Blackening Gradient.                                               (E)   (H)    (J)     (T)   (U)   (V)   (W)   (X)                              Rnd   Ct     Wht     Blk   Smth  Seg   Seg   BlSeg                            Col   Vec    Vec     Sig   BSig  #     Mass  Cntrd                            ______________________________________                                        0     1      0       0.0   0.0   --        0                                  1     1      0       0.0   0.0             0                                  2     1      0       0.0   0.0             0                                  3     1      0       0.0   0.1             0                                  4     2      1       0.5   0.4             0                                  5     2      1       0.5   0.6             0                                  6     2      2       1.0   0.9   1     3.1    0*                              7     2      2       1.0   0.8               0                                8     3      0       0.0   0.3               0                                9     3      0       0.0   0.0   --        0                                  10    3      0       0.0   2.0             0                                  11    3      24      8.0   6.0             0                                  12    3      24      8.0   9.0             0                                  13    3      36      12.0  11.0  2     45.1  1                                14    3      36      12.0  10.0              0                                15    3      6       2.0   4.5               0                                16    3      6       2.0   1.6               0                                17    3      1       0.3   0.7               0                                18    3      1       0.3   0.2               0                                19    3      0       0.0   0.1   --        0                                  20    3      0       0.0   0.03            0                                  21    2      2       1.0   0.8             0                                  22    2      2       1.0   0.9   3     3.2    0*                              23    2      1       0.5   0.6               0                                24    2      1       0.5   0.4               0                                25    1      0       0.0   0.1               0                                26    1      0       0.0   0.0   --        0                                  27    1      0       0.0   3.8             0                                  28    1      15      15.0  11.3  4     26.4  1                                29    1      15      15.0  11.3              0                                ______________________________________                                         KEY                                                                           (T): Normalized blackening gradient signal = blackening gradient vector /     count vector                                                                  (U): Smoothed normalized blackening gradient signal = (previous pixel + (     × pixel) + next pixel)/4                                                (V): Segment number                                                           (W): Segment mass                                                             (X): Weighted centroid of blackening gradient segment                         *segment rejected because of low mass                                    

After means 1510 normalizes the gradient vectors, means 1512 smoothesthe resulting whitening and blackening gradient signals. The smoothingfilter employed by means 1512 preferably depends on the oversamplingrate used to stretch the scan line data. Table IX presents the preferredfilter parameters for different oversampling rates.

                  TABLE IX                                                        ______________________________________                                        Oversampling    Filter                                                        Rate            size    Weights                                               ______________________________________                                        1, 2            1       1                                                     3, 4            3       1,2,1                                                 5, 6            5       1,2,4,2,1                                             >= 7            7       1,2,4,8,4,2,1                                         ______________________________________                                    

List (P) of Table VII presents the results of smoothing the whiteninggradient signal of list (O) based on an oversampling rate of 3. For anoversampling rate of 3, each element S_(i) in the smoothed whiteninggradient signal of list (P) is calculated using: ##EQU2## where W_(i) isthe element in the whitening gradient signal of list (O) correspondingto element S_(i), W_(i-1) is the previous element in list (O), andW_(i+1) is the next element in list (O). Similarly, list (U) of TableVIII presents the results of smoothing the blackening gradient signal oflist (T) based on an oversampling rate of 3.

After means 1512 smoothes the two gradient signals, means 1514 transmitsthe smoothed gradient signals to gradient signal processor 114 of system100. Those skilled in the art will understand that the smoothing ofmeans 1512 is an optional processing step.

Processing and Decoding the Smoothed Gradient Signals

Referring now to FIG. 16, there is shown a block flow diagram of theprocessing implemented by gradient signal processor 114 of system 100 toprocess the smoothed whitening and blackening gradient signals generatedby generator 112. The whitening and blackening gradient signalsrepresent the locations of dark-to-bright and bright-to-darktransitions, respectively, in the bar code symbol. That is, theyrepresent the trailing and leading edges, respectively, of bars in thebar code symbol. Processor 114 locates the bright/dark transitions inthe gradient signals and generates a single one-dimensionalreconstructed signal from those transition locations. Processor 114transmits this reconstructed signal to gradient signal decoder 116,which decodes the signal using conventional bar code decoding methods.

In a preferred embodiment, processor 114 segments the two gradientsignals, locates the centroids of the signal segments, and reconstructsa single binary signal from the centroid locations. In an alternativepreferred embodiment (not shown), processor 114 selects the peaks in thetwo gradient signals as the locations of the transitions for thereconstructed signal.

Means 1602 of FIG. 16 receives the smoothed whitening and blackeninggradient signals from generator 112 and segments them into individualsegments. Each segment begins with a local minimum in the gradientsignal and ends with the next local minimum. Each segment, therefore,contains a single local maximum. Referring again to Table VII, list (Q)identifies the segments in the smoothed whitening gradient signal oflist (P). Similarly, list (V) of Table VIII identifies the segments inthe smoothed blackening gradient signal of list (U).

To minimize erroneous segmentation, means 1602 preferably uses asignificance-value test to determine the local minima that define thesegments in the gradient signals, where the significance-value test issimilar to that described earlier in this specification in conjunctionwith FIG. 11.

After means 1602 segments the smoothed gradient signals, means 1604determines the mass of each segment. The mass of each segment is the sumof the smoothed gradient values for that segment. List (R) of Table VIIpresents the masses of the whitening gradient segments identified inlist (Q). Similarly, list (W) of Table VIII presents the masses of theblackening gradient segments identified in list (V). Thus, for example,segment #3 in Table VII has a mass of (0.1+0.2+0.2+0.1) or 0.6.

After means 1604 determines the mass of each segment, means 1606 locatesthe weighted centroid of those segments that have sufficient mass. Means1606 ignores those segments whose masses are less than a specified massthreshold, indicating that they have insufficient mass. The massthreshold is preferably specified to be equivalent to a fraction, forexample, one-eighth, of the product of the oversampling rate and theexpected mass of a narrow bar or space in the original pixel image.

Noise in the pixel image may result in spurious segments in the gradientsignals. If the image noise is not too great, the spurious segments willhave masses typically lower than the masses of true segments (i.e.,those corresponding to true bright/dark transitions in the pixel image).In that case, means 1606 may be used to "filter" out the spurioussegments.

In Table VII, segments #1, 2, 3, and 5 are rejected for having massesless than the specified mass threshold of 20. Segments #1 and 3 of TableVIII are also rejected for similarly having masses less than the massthreshold.

After eliminating the segments having low mass, means 1606 locates theweighted centroids of the remaining segments. The weighted centroid of asegment is the location within the segment corresponding to a specifiedpercentage of the mass of the segment. For example, a weighted centroidbased on 50% corresponds to the center of mass of each segment. Aweighted centroid based on 40% corresponds to the location within thesegment wherein 40% of the mass is to the left and 60% of the mass is tothe right.

In Table VII, a "1" in list (S) identifies the location of the weightedcentroid of segment #4, where 50% is the selected weighted-centroidpercentage. Similarly, list (X) of Table VIII identifies the locationsof the weighted centroids of segments #2 and 4.

In an alternative preferred embodiment (not shown), differentweighted-centroid percentages are used to locate the centroids in thewhitening and blackening gradient segments. Since black bars tend to"bleed" into white spaces in bar code symbol images, bars tend to appearwider and spaces narrower in the images. Using a whitening-centroidpercentage that is smaller than the blackening-centroid percentage maycorrect these bleeding effects. The exact weighted-centroid percentagesmay be selected empirically based on tests performed on images of knownbar code symbols.

After means 1606 locates the whitening and blackening segment centroids,means 1608 reconstructs a single one-dimensional signal by interleavingthe centroid locations from the two gradient signals. The centroid ofeach segment from the smoothed blackening gradient signal corresponds tothe leading edge of a bar in the bar code symbol. Similarly, thecentroid of each segment from the smoothed whitening gradient signalcorresponds to the trailing edge of a bar in the bar code symbol.

Table X presents the results of means 1608 interleaving the whiteningand blackening segment centroids of Tables VII and VIII. Lists (E), (S),and (X) of Table X are identical to the corresponding lists of TablesVII and VIII. List (Y) of Table X represents the one-dimensionalreconstructed signal generated by means 1608. Note that thereconstructed signal of list (Y) of Table X is identical to thethresholded signal of list (N) of Table VI, indicating that the gradientsignal processing of generator 112 and processor 114 of system 100generates the same result as the composite signal processing generator900 and thresholder 108 of system 100.

                  TABLE X                                                         ______________________________________                                        Reconstructing Segmented Signal.                                              (E)      (S)           (X)     (Y)                                            Rnd      WhSeg         BlSeg   Rec                                            Col      Cntrd         Cntrd   Sig                                            ______________________________________                                        0        0             0       1                                              1        0             0       1                                              2        0             0       1                                              3        0             0       1                                              4        0             0       1                                              5        0             0       1                                              6        0             0       1                                              7        0             0       1                                              8        0             0       1                                              9        0             0       1                                              10       0             0       1                                              11       0             0       1                                              12       0             0       1                                              13       0             1       0                                              14       0             0       0                                              15       0             0       0                                              16       0             0       0                                              17       0             0       0                                              18       0             0       0                                              19       1             0       1                                              20       0             0       1                                              21       0             0       1                                              22       0             0       1                                              23       0             0       1                                              24       0             0       1                                              25       0             0       1                                              26       0             0       1                                              27       0             0       1                                              28       0             1       0                                              29       0             0       0                                              ______________________________________                                         KEY                                                                           (Y): Reconstructed signal                                                

If the reconstructed signal has two consecutive leading bar edges, theneither a trailing bar edge was missed or an extra leading bar edge wasinserted. Similarly, if the reconstructed signal has two consecutivetrailing bar edges, then either a bar leading edge was missed or anextra bar trailing edge was inserted. If true bar edges are beingmissed, then the segmentation procedure performed by means 1602 shouldbe made more sensitive to smaller gradients. If spurious bar edges arebeing inserted, then the segmentation procedure should be made lesssensitive to accommodate larger changes in the gradient signal withoutfalsely splitting a segment in two. The sensitivity of the segmentationprocedure may be controlled by varying the specified mass threshold usedby means 1606 to determine whether segments have sufficient mass.

After means 1608 generates a reconstructed signal, means 1610 transmitsthe reconstructed signal to gradient signal decoder 116. In a preferredembodiment, the processing of decoder 116 is identical to that ofcomposite signal decoder 110 described earlier in this specification inthe section entitled "Decoding Composite Signals." After decoding thebar code symbol, decoder 116 transmits the decoded signal to outputselector 118.

As described earlier in this specification in conjunction with theexample of Table II, under a preferred embodiment of the presentinvention, the original pixel intensity values are repeated to "fill inthe gaps" between the rounded columns that result from rotating,stretching, and rounding the original columns. In an alternativepreferred embodiment (not shown), the original pixel intensity valuesare repeated according to the oversampling rate.

For example, referring to Table III, for an oversampling rate of three,system 100 would rotate and stretch original pixel (0,0) such that theintensity value of pixel (0,0) would be used three times to update theelements of the composite vector corresponding to rounded columns 0, 1,and 2. Similarly, system 100 would rotate and stretch original pixel(0,1) such that the intensity value of pixel (0,1) would be used threetimes to update the elements of the composite vector corresponding torounded columns 2, 3, and 4.

Note that the intensity values of both original pixels (0,0) and (0,1)of scan line #1 are used to update the element of the composite vectorcorresponding to rounded column 2. Thus, rather than "filling in thegaps" between rounded columns, in this preferred embodiment, theoriginal pixel values are repeated according to the oversampling rateand "overlapping" may result. In this example, the element of the countvector corresponding to rounded column 2 is updated two times to reflectthis overlapping effect.

System 100 may be implemented in hardware, software, or a combination ofhardware and software. In a preferred embodiment, system 100 isimplemented in software running on common microprocessors such asIntel's 80X86 family or Motorola's 680X0 family.

Those skilled in the art will understand that preferred embodiments ofsystem 100 may be designed to locate and decode bar code symbols inbinary and/or gray-scale images.

Those skilled in the art will also understand that system 100 ispreferably designed to be parameter-driven, where the values for variousparameters may be changed depending on the particular application. Itwill also be understood that the selection of certain parameter valuesmay be based on empirical analysis from processing known sample images.

It will be further understood that various changes in the details,materials, and arrangements of the parts which have been described andillustrated in order to explain the nature of this invention may be madeby those skilled in the art without departing from the principle andscope of the invention as expressed in the following claims.

What is claimed is:
 1. A method for reading a bar code symbol in a pixelimage, comprising the steps of:(a) selecting a first scan linesubstantially parallel to the bars and spaces of said symbol; (b)scanning said symbol along said first scan line to generate a firstvalue based on intensity values of two or more pixels of the pixel imagethat are crossed by the first scan line; (c) selecting a second scanline substantially parallel to the bars and spaces of said symbol,wherein the distance between said first and second scan lines is lessthan the width of a pixel of said image; (d) scanning said symbol alongsaid second scan line to generate a second value based on intensityvalues of two or more pixels of the pixel image that are crossed by thesecond scan line; and (e) decoding said symbol in accordance with saidfirst and second values.
 2. The method of claim 1, wherein said firstvalue comprises the average of intensity values of two or more pixelslying along said first scan line.
 3. The method of claim 1, wherein step(a) further comprises the step of locating the four corners of saidsymbol, and wherein said first and second scan lines are selected inaccordance with said four corners.
 4. The method of claim 1, whereinstep (e) comprises the steps of:(1) generating a composite signal inaccordance with said first and second values; (2) thresholding saidcomposite signal; and (3) decoding said symbol in accordance with saidthresholded composite signal.
 5. An apparatus for reading a bar codesymbol in a pixel image, comprising:(a) means for selecting a first scanline substantially parallel to the bars and spaces of said symbol; (b)means for scanning said symbol along said first scan line to generate afirst value based on intensity values of two or more pixels of the pixelimage that are crossed by the first scan line; (c) means for selecting asecond scan line substantially parallel to the bars and spaces of saidsymbol, wherein the distance between said first and second scan lines isless than the width of a pixel of said image; (d) means for scanningsaid symbol along said second scan line to generate a second value basedon intensity values of two or more pixels of the pixel image that arecrossed by the second scan line; and (e) means for decoding said symbolin accordance with said first and second values.
 6. The apparatus ofclaim 5, wherein said first value comprises the average of intensityvalues of two or more pixels lying along said first scan line.
 7. Theapparatus of claim 5, wherein means (a) further comprises means forlocating the four corners of said symbol, and wherein said first andsecond scan lines are selected in accordance with said four corners. 8.The apparatus of claim 5, wherein means (e) further comprises:(1) meansfor generating a composite signal in accordance with said first andsecond values; (2) means for thresholding said composite signal; and (3)means for decoding said symbol in accordance with said thresholdedcomposite signal.